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2005 Ford Transmission PRNDL Logic Pulse... need help.

Discussion in 'Electronic Basics' started by Thomas, Apr 1, 2005.

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  1. Thomas

    Thomas Guest

    I am trying to activate a relay when the vehicle is in anything other
    than Park. The transmission module sends out 5V @ 120hz with varying
    pulse widths. In Park, I read 0.71V with the meter and looks to be
    apprx 1/7th of the pulse on the scope (square wave). In Reverse, the
    pulse is measured at 1.6V of 1/3 of the pulse with higher voltages and
    pulse widths in N,D,2. I've tried using this to enable a small dc
    relay, but only get chatter (even in Park). I've been told I could do
    the following...

    RC Integrator - Unity-Gain OpAmp Buffer - Comparator

    How would this work? Any help is greatly appreciated.
  2. Andrew Holme

    Andrew Holme Guest

    The RC integrator will smooth-out the pulses, giving you a DC voltage
    proportional to the pulse width.

    Your meter is doing the same thing i.e. reading the average level.

    The comparator gives you a digital output indicating whether the voltage is
    above or below some threshold. You'll need positive feedback (hysteresis)
    around the comparator to cope with residual ripple.

    There are two tweak-factors to play with:
    1. RC time constant
    2. Amount of hysteresis

    Long time constant = more smoothing = slower response to change
    Short time constant = fast response, but more residual ripple on DC voltage
    = more hysteresis required

    Given the application, a time constant of up to 1 second should be OK, and
    would provide effective smoothing at 120Hz.

    You won't need a unity gain op-amp buffer if the RC filter output is the
    only thing connected to the inverting input of the comparator; and the
    threshold preset, and postive feedback resistor, are connected to the
    non-inverting input.
  3. Thomas

    Thomas Guest

    Is there any way you could assist in the selection of the correct
    components or point me in the right direction as to the right RC
    Integrator? Thanks for the input!
  4. Andrew Holme

    Andrew Holme Guest

    The time constant depends on the product R*C. Many different value pairs
    are possible; however, you might start with (say) 100k and 10uF. Look at
    the signal on an oscilloscope and experiment with the values. Large values
    for R will minimise loading of the input signal. You want the output to be
    a steady DC level with minimal ripple. Increasing R*C will reduce ripple.
    By minimal ripple, I mean relatively small (say <5%) compared to the level
    changes produced by moving the gear selector. This enables the comparator
    to distinguish between ripple and gear shifts.
  5. Jamie

    Jamie Guest

    you need a Window Comparator. the output of the comparator
    can operate a Time Off Delay circuit. each time it triggers
    it will recharge the Time off or initially start it.

    the whole thing could be built around a simple thing like a
    LM324 which has 4 op-amps in one package.
    2 of them used as the window comparator.
    #3 a slight Time on delay buffer, #4, A time off Delay comparator.

    the time off delay is tailured to hold the output steady between
    your 120Hz Rates.

    look on the net for "Window Comparator"
    and also look for using a OP-Amp as a time delay and time off
  6. John Fields

    John Fields Guest

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